This invention relates generally to press rolls and more particularly to twin roll presses having rolls with features for minimizing rewet of a fibrous pulp cake after it has passed through a roll nip in which liquid is expressed from the fibrous pulp.
Twin roll presses are used to thicken pulp slurries from approximately 4 percent consistency to between 30 and 50 percent consistency (measured as percent dry fiber in a given weight of slurry). Such presses are well-known in the art and are applied, for example, to dewatering of papermaking pulp and to washing such pulp followed by such dewatering.
Two rolls having perforated decks are installed side by side in a pressurized vat into which a pulp slurry is fed from both longitudinal sides of the vat. The rolls are counterrotating so that their outer edges travel downward into the slurry and their inner edges travel upwardly to define a nip between the rolls. A pulp mat which forms on the surface of the rolls, due to flow of liquid through the porous roll decks, is squeezed at high pressure in the nip to express a substantial portion of the remaining liquid from the pulp. After passing upward through the roll nip, the relatively dry pulp cake is scraped from the rolls by doctor blades and is conveyed out of the vat at a substantially higher consistency than the feed consistency.
Generally, the rolls consist of an axial core upon which are arrayed a number of longitudinal support ribs which support, at their outer edges, a heavy walled hollow roll shell. The roll shell has a number of circumferential grooves within which are drainage holes providing liquid communication between the grooves and a number of internal drainage compartments defined by the outer surface of the core, the support ribs, and the inner surface of the shell. Drainage slots are provided in each support rib adjacent the inner surface of the roll shell to permit extracted liquid to flow to the lowest drainage compartment within the roll deck. Actual flow of the liquid from the roll deck drainage compartments is out the ends of the roll.
Ideally, the filtrate extracted through the perforated roll surface is drained at a rate sufficient to assure that the drainage compartments are liquid free when such compartments emerge above the nip. However, at practical production rates, this is rarely accomplished. As a result, some of the liquid remaining in the drainage compartments drains back through the roll surface and is absorbed by the expanding pulp mat. This results in the pulp being discharged at a consistency several points below the peak consistency achieved in the roll nip.
As capacity demands increase, longer rolls, higher roll speed and increased filtrate flow will be required. In addition, higher consistency is always desired and demands increased nip load. If the thicknesses of the roll shell, the support ribs and the core are increased to provide the added support required for the increased nip loads, significantly smaller flow passages will result for the same roll diameter. The increased production will require a greater volume of filtrate to flow through these smaller passages. It is to be expected, therefore, that the degree of rewetting will increase. In the case of a wash press, such rewetting with dirty liquid seriously lowers the washing efficiency.
The foregoing illustrates limitations known to exist in present devices and methods. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.